The present invention relates to a projection type image display apparatus of the type which is used as a projection TV set, etc., in which white light from a light source is separated into additive primary colors, each of the primary colors is modulated with the use of a display element, and images are expanded and displayed on a screen.
Along with a diversity of video sources, projection type image display apparatuses are popular as optical projection apparatuses for a large screen as a result of its marketable properties, such as lightness in weight, low price, and compactness in size. In particular, the projection type image display apparatuses using a liquid crystal display element (hereafter, referred to as a liquid crystal panel) as a video generation source has come onto the market because of recent significant improvement of the definition and numerical aperture of a liquid crystal panel. Unlike the conventional projection type CRT, the liquid crystal panel does not emit light by itself, so it needs a light source. The projection type image display apparatus with a liquid crystal panel is composed so that a white light from its white light source is separated into additive primary colors and each of those primary colors are modulated in the liquid crystal panel, from which full-color images are displayed on the screen by expanding original images on the liquid crystal panel through a projection lens unit.
The optical system of the projection type image display apparatus that employs this liquid crystal panel is divided into two types, i.e. a three-panel type that uses three liquid crystal panels and a single-panel type that uses only one liquid crystal panel.
The three-panel type optical system has a liquid crystal panel and an optical unit (color separator) for each respective color of the primary colors (red, green, and blue) obtained by separating white light. The optical unit (color separator) propagates one of the obtained primary colors and the liquid crystal panel modulates the intensity of the colored light to form an image. Each color image is superposed with the other color images optically (color synthesizer) so as to display an image in full colors. This three-panel configuration of the optical system has advantages in that the light from the white light source can be used effectively to obtain high purity colors. In spite of this, because the optical system requires both a color separator and a color synthesizer as described above, the number of parts is increased in the optical system and, accordingly, the cost becomes higher than that of the single-panel configuration.
On the other hand, the single-panel configuration of the optical system uses only one liquid crystal panel, and it is divided into two types according to how TFT apertures are disposed in itself; delta type and stripe type. In the early single-panel configuration, a color filter was used to separate a white color into additive primary colors, but the configuration was plagued with the problem in practical use that the color filter absorbed and reflected the light, thereby the usage efficiency of the light was lowered to about ⅓ that of the three-panel configuration.
In order to solve this problem, for example, the Japanese Patent Unexamined Publication No.4-60538 has disclosed a single-panel color liquid crystal display apparatus, which, as shown in FIG. 1 thereof, employs dichroic mirrors 4R, 4G, and 4B disposed in a fan-like pattern so as to separate white color light obtained from a white color light source 1 into red, green, and blue light fluxes, thereby improving the usage efficiency of the light.
In this apparatus, each of the light fluxes R, G, and B separated by the above dichroic mirrors 4R, 4G, and 4B is injected at a different angle from the others into a micro-lens array 10 disposed at the light source side of a liquid crystal display element 20 shown in FIG. 2 in the above-referenced publication.
Each light flux passing this micro-lens array 10 is distributed and irradiated at a liquid crystal site driven by a signal electrode to which a color signal corresponding to one of those light fluxes is applied. Consequently, the usage efficiency of the light is greatly improved, thereby obtaining brighter images than the liquid crystal display element that employs an absorption type color filter.
The official gazette of Japanese Patent Laid-Open No.5-328805 has also disclosed a projection type color liquid crystal display apparatus that has improved color purity by minimizing the generation of stray lights by starting the separation of white color light into the additive primary colors at the long wavelength side so as to prevent color mixing caused by the angle dependency of the wavelength selection characteristics of each of the dichroic mirrors. According to this method, because the original light is separated into light fluxes in the order of R, G, and B, thereby shifting the characteristics of each dichroic mirror, stray lights are not generated easily and the color purity of each separated light flux is improved. Images can thus be projected at a wide range of color reproduction.
However, in the technique disclosed in the above-referenced publication where the angle xcex1 is obtained when the G light flux is injected at an angle close to the normal of the liquid crystal display element, as shown in FIG. 6(a) thereof, and is diffracted by a micro-lens and the angle xcex2 is obtained when each of the R and B light fluxes is injected obliquely to the normal of the liquid crystal display element, as shown in FIG. 6(b) thereof, and is diffracted by a micro-lens; the angle xcex2 is larger than the angle xcex1 of the light flux (G) irradiated from the liquid crystal display element. This requires a large diameter (low F value) projection lens, thus becoming a primary factor for increasing the manufacturing cost of the projection type color display apparatus.
In order to solve this problem, the Japanese Patent Unexamined Publication No. 8-114780 disclosed a method for keeping a favorable white balance with the use of a small diameter projection lens by injecting a color light emitted from the light source with the weakest spectrum at an angle close to the normal of the liquid crystal display element, thereby eliminating the eclipse at the pupil of the projection lens with the least volume color light.
Because the purity of the color light with the least light volume is improved, it is possible to obtain a wider color reproduction range and more clear images.
One of the projection lenses used for the optical system of the projection type image display apparatus described above is a retrofocus lens of the type disclosed, for example, in the Japanese Patent Unexamined Publication No.9-96759. (Because of the long flange back, it is the most suitable for the three-panel configuration of the optical system.) Because the half-angle of view of this projection lens is about 42xc2x0, the projection distance is short. If it is employed for a back-projection type image display apparatus, therefore, the arrangement will be more compact in size even when only one reflection mirror is employed.
Generally, the transmission type screen used in this case employs a two-panel configuration consisting of a lenticular sheet and a Fresnel lens sheet. In some cases, the transmission type screen is also provided with a lenticular lens on the image light injection surface of the Fresnel lens sheet so that the lenticular lens is shaped so as to be longer in the horizontal direction of the screen.
However, in the single-panel configuration described above it is difficult to obtain a predetermined purity for each color. Only with the means proposed in the Japanese Patent Unexamined Publication No. 8-114780. This is because, according to this method, each of the R, G, and B light fluxes separated by a dichroic mirror is injected at a different angle from the others into the micro-lens array 7 disposed at the light source side of the liquid crystal display element shown in FIG. 7 thereof. Each light flux passing this micro-lens array 7 is distributed and irradiated on the liquid crystal sites 24G, 24R, and 24B driven by a signal electrode respectively to which a color signal corresponding to each color light flux is applied independently. At this time, each junction between those micro-lenses provided at the micro-lens array 7 is not formed sharply, thereby it disperses the light. Consequently, for example, part of the green light flux, whose relative visibility is the highest and whose emission spectrum from the light source is dispersed at the junction, is then mixed into the red light flux whose emission spectrum from the light source is the weakest. Thus, the red color purity is lowered at the liquid crystal site 24R due to the mixture of the red light flux and the green light flux. The liquid crystal site 24R is originally injected only with the red light. This is why each color purity cannot reach its predetermined value with the above method.
If the reflection characteristics of the dichroic mirror for separating the red color are set so as to improve the purity thereof, however, the light volume of the red light flux to be obtained is reduced, thereby the white balance obtained by adding the three primary colors is lost.
At this time, if the white balance is adjusted by reducing the light volume of each of the other two color lights, then the luminance of the white video obtained by adding the three primary colors is lowered.
As described above, even in the case of the projection type color liquid crystal display apparatus proposed in the Japanese Patent Unexamined Publication No. 8-114780, both the brightness and the color purity are not able to reach satisfactory levels when compared with those of the projection type display apparatus that employs a conventional projection type CRT. In addition, because the luminance level is high when images are displayed in black on the liquid crystal panel, the contrast of the images becomes unfavorably low.
On the other hand, in order to realize a compact rear projection type image display apparatus for general home use, the projection distance (distance between the projection lens unit and the screen) must be reduced. Thus, a wider projection lens unit is required. At this time, if an ordinary wide projection lens unit is used for the apparatus, the peripheral light volume ratio is reduced significantly due to the light distribution characteristics of the liquid crystal panel. This is because the spectrum transmittance and reflectance of each of the three dichroic mirrors disposed between the liquid crystal panel and the white light source differs among injection angles of the light, so that the light flux from the white light source is injected into each dichroic mirror and the liquid crystal panel. As a result, the main light beam injected into the projection lens unit from each object point of the liquid crystal panel goes approximately in parallel to the light axis of the projection lens unit and the distributed angle becomes proportional to the numerical aperture of the micro-lens. If a wider projection lens is employed for the optical system, then the light fluxes to be injected into the projection lens unit from around the liquid crystal panel is reduced extremely, thereby the peripheral portion of each expanded image on the screen becomes dark.
In addition to the problems described above, the above-mentioned method is also confronted with the following problems that must be solved. (1) Each image must be focused accurately in every corner. (The chromatic aberration of the magnification must also be reduced.) (2) The F value must be reduced so as to improve the brightness of the screen. (3) Because of the inability of convergence adjustment, the distortion must be reduced. (4) The reflection on the lens surface must be reduced, to the extent of suppressing the loss of brightness and securing the contrast property sufficiently.
As described above, the projection lens units proposed to data have many problems that must be solved. Actually, however, even the retrofocus lens proposed in the Japanese Patent Unexamined Publication No. 9-96759 cannot secure enough brightness because of the large F value (2.56) and the shorter projection distance while the half-angle of view is about 42xc2x0.
The conventional optical projection system that employs a liquid crystal panel is also provided with a normal white light source and a cooling fan for cooling the liquid crystal panel (including a polarizing plate). Consequently, the cost of the optical system is increased and the reduction of the blowing sound has been a problem that must be solved. In the case of the air-cooling method, it is difficult to cool down the polarizing plate satisfactorily. The polarizing plate is thus affected by the heat and experiences a change in physical properties, thereby deteriorating the polarization degree and the contrast.
On the other hand, the transmission type screen used for the apparatus is manufactured by the conventional technique proposed in the Japanese Patent Unexamined Publication No. 58-59436. According to the conventional technique, the lenticular lens disposed on the injection surface is part of an elliptic cylindrical surface and the ellipse is formed so that the long axis is assumed in the direction of thickness between the injection surface and the ejection surface, and one of the two focal points of the ellipse is positioned inside the substrate and the other focal point is positioned around the ejection surface. In addition, the eccentricity of the ellipse is selected so as to take an approximate inverse number of the refractivity of the base material.
As a result, if a light flux in parallel to the long axis of the ellipse is injected in the injection surface, the light beam goes into aberration entirely at the focus around the ejection surface, causing the light beam to be dispersed from this focal point in the horizontal direction of the screen.
On the other hand, the lenticular lens provided on the ejection surface has an elliptic cylindrical surface formed almost symmetrical to the elliptic cylinder on the injection surface. The actual lenticular lens sheet does not cause the light to be focused at a point, but is dispersed, since a dispersion material is mixed in the lens sheet, as shown in FIGS. 31 and 32 thereof. Consequently, it is impossible to increase the width of the light absorption layer in the horizontal direction of the screen by more than the width of the lenticular lens. The reflected light caused by an external light cannot be reduced and the reduction of the contrast cannot be suppressed within a fixed value.
The above descriptions can thus be summarized as follows. The rear projection type image display apparatus that employs a single-panel optical projection system is confronted with new problems that have never been found in the rear projection type image display apparatus that employs a conventional CRT. The problems are: (1) The focus property must be further improved. (2) The contrast property must be further improved. (3) The requirements of both color purity and brightness must be satisfied.
In order to solve the above first problem, the projection lens unit of the present invention is composed so that a plurality of lens elements for projecting an expanded image of light received from an image generation source on a screen are disposed along the light axis. In this regard, first to third lens groups are disposed in order from the screen side. The first lens group has a negative refractive power as a whole, the second lens group has a positive refractive power as a whole, and the third lens group has a negative refractive power as a whole and includes at least a lens element having a negative refractive power at its center portion and a positive refractive power at its peripheral portion. The first lens group is composed so as to include at least a meniscus lens provided with a convex surface facing the screen and having a negative refractive power. The second lens group may be composed so as to include at least a lens having a negative refractive power, which is obtained by combining a double-convex lens having the first Abbe number and a double-convex lens having the second Abbe number which is smaller than the first Abbe number. Furthermore, the second lens group also includes a lens element having a positive refractive power at its center portion including the light axis and having almost no refractive power at its peripheral portion away from the light axis in the radial direction or having a negative refractive power there.
The projection lens unit for achieving a first object of the present invention, as described above, comprises the first lens group having a negative refractive power, the second lens group having a positive refractive power, and the third lens group having a negative refractive power. Those three lens groups are disposed in order from the screen side. This configuration can obtain a flat surface for each image even when the angle of view is 80xc2x0 or over, so images can be focused favorably in every corner. Furthermore, because the first and third lens groups having a negative refractive power respectively are disposed at both sides of the second lens group having a positive refractive power in this configuration, it is not only advantageous to correct the field curvature, but also effective to suppress the distortion of images.
The projection lens unit in the three-group configuration, however, comes to have a problem in that the first and third lens groups have large diameters, increasing the manufacturing cost. In order to avoid this problem, therefore, the projection lens unit of the present invention is provided with a lens which is aspheric in shape so as to have a negative refractive power (for dispersing) around the light axis and a positive refractive power at its peripheral portion. The lens is disposed in the third lens group, thereby suppressing the diameter of the lens while making effective use of the basic configuration described above.
The second lens group is provided with an aspheric lens having a positive refractive power (for condensing) around the light axis and a negative refractive power or almost no refractive power at its peripheral portion (for dispersing or almost no effect for dispersing). The second lens group is combined with the third lens group as described above, thereby having the optical system function as a beam expander (for changing the width of the light flux) which can compress each light flux from the liquid crystal panel in the axial direction of the light. As a result, the effective height of the object surface can be reduced, thereby. making it easier to correct the aberration including the magnification color aberration.
Furthermore, the second method for achieving the above first object involves canceling of both single color aberration and magnification color aberration caused by the red and blue light fluxes by optimizing the refraction and dispersion of each lens element included in the second lens group. The projection lens unit in this configuration can secure a high focusing property and a sufficient peripheral light volume ratio. This is because the telecentric configuration is taken so that the main light beam goes almost in parallel to the light axis of the projection lens unit and the ejection pupil through which the light flux focused at the periphery of the screen passes becomes larger than the ejection pupil on the light axis.
It is thus clear that the light flux can be compressed in the radial direction of the light if an aspheric lens element, which has a negative refractive power around the light axis (for dispersing the light) and a positive refractive power at its peripheral portion (for condensing), is disposed at a position nearest to the liquid crystal panel. This effect can also be obtained with any device if its light flux ejected from the liquid crystal panel, which is an object point, is almost in parallel to the light axis. There is thus no need to use a lens unit provided with three lens groups disposed so as to nave a negative refractive power, a positive refractive power, and a negative refractive power in order from the screen side.
In other words, if a light flux is compressed so as to minimize the diameter of a lens in a projection type image display apparatus that employs a liquid crystal panel, it will be effective to dispose an aspheric lens element at a position nearest to the liquid crystal panel. The lens element should have a negative refractive power (for dispersing) around the light axis and a positive refractive power (for condensing) at its peripheral portion.
Furthermore, in order to focus images clearly at any part of the screen so as to obtain brighter images, the projection lens unit of the present invention provides an aspheric lens at a position where the light flux formed in the center of the screen is not overlapped with the light flux to be formed at the outermost periphery of the screen. A low-price plastic lens may be used as the aspheric lens if a mass production is possible for the lens. However, this plastic lens experiences a problem in that the refractive power is changed according to changes of the shape and refractivity due to temperature changes and moisture absorption. Accordingly, the focal point is changed and the focusing property is degraded. In order to avoid the problem, the present invention takes the following two measures for the configuration of the projection lens unit. (1) The thickness of the plastic lens is unified as much as possible, thereby reducing the change of the refractive power to be caused by changes of the shape and refractivity due to temperature changes and moisture absorption. (2) A plurality of plastic aspheric lenses are combined to counterbalance the variation of the refractive power which may occur in response to temperature and humidity changes caused by the change of the local shape of the plastic aspheric lens.
Furthermore, a third method for achieving the first object of the present invention makes it possible to improve the focusing property of the lens unit by devising a lighting system. The lighting system of the present invention separates white light into the additive primary color light fluxes in the order of red, blue, and green with the use of dichroic mirrors, then each of those light fluxes is injected into one and the same liquid crystal panel at an angle different from the others. Consequently, the three primary color light fluxes modulated by the liquid crystal panel are separated in the horizontal direction of the screen of the liquid crystal panel when passing through the injection pupil of the projection lens unit. This is why the dichroic mirrors are used to separate the white light flux into the three primary color light fluxes so that the blue light flux passes the center of the injection pupil. The blue light-flux has the largest color aberration which occurs when the flux passes around the injection pupil. In addition, the orientation (code) of the aberration to be caused by the red light flux is corrected so as to cancel the magnification color aberration (deviation of the focal point between green and red light fluxes).
Next, technical means for achieving the second object of the present invention will be described. In this case, it is premised that the technique employed for the projection lens unit of the present invention is also used here.
The second method is to reduce the reflection loss on both the lens element composing the projection lens unit and the screen by p-polarization of the light fluxes injected into the transmission type screen from the optical projection apparatus of the present invention.
The third method is to dispose dichroic mirrors for separating white light received from a white light source used in the lighting system into three primary color light fluxes, then injecting each of those light fluxes into the liquid crystal panel at an angle different from the others in the order of a dichroic mirror for transmitting cyan (blue and green), a dichroic mirror for transmitting yellow (green and red), and a dichroic mirror for transmitting red, disposed sequentially from the white light source side. At this time, both brightness and color purity are taken into consideration to determine the optimal value of the wavelength, which reaches not less than 50% of the reflectance of each dichroic mirror.
The fourth method is to increase only a predetermined component to deflect by, about 50% by disposing a deflecting beam splitter between the white light source and the display element so as to combine polarized light fluxes. At this time, only the p-polarized wave components are taken out, thereby reducing the reflection loss in the multi-lens array composed of a plurality of lens elements. In addition, the dichroic mirrors described above and a light path reflection mirror are disposed at positions crossing the polarized beam splitter at right angles, respectively, so as to be p-polarized respectively. Consequently, the reflectance of the light path reflection mirror is increased and, accordingly, the brightness of the images is increased more.
The fifth method is to separate the white light flux to red, blue, and green light fluxes in the order of the weakness of the spectral energy distribution of the white light source when light fluxes separated by the respective lenses disposed in the first multi-lens array close to the white light source are expanded by a lens disposed so as to face the second multi-lens array positioned at the liquid crystal panel side, then the light fluxes are projected in the liquid crystal panel. As a result, the light path between the second multi-lens array and the liquid crystal panel makes the red light flux shortest, so the projection magnification of the red light flux is reduced, thereby the out-of-focus error caused by aberration occurs less and the energy density of the red light flux is increased.
Furthermore, because the blue light flux has a low relative visibility and the light path is provided with a filter for reflecting the ultraviolet ray output from the white light source in itself, the energy of the blue light flux used effectively is also reduced. This is why the blue light flux is separated from the white light flux just after the red light flux so that the blue light flux passes the center of the injection pupil of the projection lens unit as described above. The brightness of both white light and each of the three primary colors can thus be maximized when the three additive colors are displayed on the screen.
Furthermore, the first method for achieving the third object of the present invention described above is to provide the above projection type image display apparatus with a liquid crystal panel and a polarizing plate and to fill a cooling liquid in a space formed between the liquid crystal panel and a lens element of the projection lens unit, closest to the liquid crystal panel. The liquid crystal panel and the polarizing plate disposed in front of the liquid crystal panel change their physical characteristics due to a heat when the temperature rises (to 70xc2x0 C. or so), causing the polarizing characteristics to be degraded, thereby to lower the contrast property in some cases. In the case of the configuration of the present invention, however, because the liquid crystal panel and the polarizing plate are cooled down by a liquid (cooling agent), the cooling efficiency is improved more than that of the air cooling method. Consequently, it is possible to prevent deterioration of the contrast property caused by the deterioration of the polarizing characteristics caused by a rise in temperature, thereby obtaining high quality images.
Furthermore, if a medium, whose refractivity to light having a 587.6 (nm) wavelength is 1.2 or over, is used as the above cooling liquid, then the reflection of the image light is further reduced, thus the contrast property is further improved.
The second method of the present invention is to provide the lens tube of the projection lens unit with an aperture, which is structured so as to pass only a light flux modulated by the liquid crystal panel and used for forming the object image and blocking other light fluxes by absorbing them so they will not pass through the aperture. Consequently, those other light fluxes not used for forming the image do not reach the screen, thereby the contrast property of the image is further improved.
Furthermore, the third method of the present invention is to provide the transmission screen with filtering characteristics for absorbing the green light emitted with the strongest spectrum from the white light source. Consequently, the screen is protected from deterioration of the contrast property of the projected images even when external lights are injected to the screen.
Finally, the first method for achieving the fourth object of the present invention is to dispose the above dichroic mirrors at optical positions orthogonal to the polarized beam splitter respectively so that the light is s-polarized to the dichroic mirrors. Consequently, the rising part of the spectral reflectivity characteristics of each dichroic mirror becomes sharp, thus each color purity is improved.
The second method for achieving the fourth object of the present invention is to separate the white light from the light source in the order of weakness (in order of red, blue, and green) in the spectral energy distribution of the white light source when each light flux separated by the corresponding lens provided in the first multi-lens array close to the white light source is expanded by a lens in the second multi-lens array, which is facing the first multi-lens array. The second multi-lens array is provided at the liquid crystal panel side so that each light flux is projected on the liquid crystal panel. Consequently, the red light flux takes the shortest way in the light path between the second multi-lens array and the liquid crystal panel, whereby the energy density of the red light flux becomes large and the color purity is improved. In addition, when the red light flux comes into the micro-lens array of the liquid crystal panel, the red light flux is not adjacent to the green light flux having the highest relative visibility and a strong emission spectrum from the light source in the same micro-lens array, part of the red light flux is dispersed at a joint between micro-lenses, so that the green light flux is not mixed easily with the red light flux having the weakest emission spectrum from the light source. The color purity is thus improved.
Furthermore, the third method for achieving the fourth object of the present invention is to reduce the ripple component of the spectral reflection factor characteristics of each dichroic mirror, thereby making it difficult to generate stray lights. Consequently, the color purity of each separated light flux is improved and images can be projected in a wider range of color reproduction.
Furthermore, none of light fluxes separated due to the improper profile irregularity at each joint of lenses provided in the first multi-lens array close to the white light source are able to enter the lenses of the second multi-lens array facing the first one provided at the liquid crystal panel side. Some of those light fluxes enter adjacent lenses. Consequently, none of expanded light fluxes enter the liquid crystal panel at a predetermined angle, thereby mixing with other colors. In addition, abnormal light is reflected on the side surfaces and/or the top and bottom surfaces of the optical projection apparatus and enters each of the dichroic mirrors. As a result, those reflected lights cause wavelength shifts, so that light fluxes other than a predetermined wavelength enter the liquid crystal panel, thereby deteriorating color purity. This is why the side surfaces, as well as the top and bottom surfaces of the optical projection apparatus are serrated, embossed or matted, thereby lowering the reflectivity thereof. In addition, a plurality of aperture diaphragms are provided at a place where light fluxes pass, thereby absorbing and blocking unnecessary light fluxes so as to reduce the amount of abnormal light fluxes which enter the dichroic mirrors and to suppress the deterioration of the color purity.
Furthermore, the fifth method for achieving the fourth object of the present invention is to provide the transmission type screen with filtering characteristics for absorbing the green light flux emitted with the strongest spectra from the white light source consequently, it is possible to reduce the green light having the strongest emission spectrum in which red and blue lights are mixed, thereby the color purity is improved for each of the other color lights.